1. Technical Field
The technology disclosed herein is related to a highly sensitive composite imaging element, and to an imaging device equipped with this composite imaging element.
2. Background Information
In the field of imaging devices, there are known imaging elements that convert light incident on a light receiving face (or surface) into an electrical signal by opto-electrical conversion, such as charge coupled device (CCD) type image sensors or complimentary metal oxide semiconductor (CMOS) type image sensors. These imaging elements are used in imaging devices such as digital cameras and digital video cameras to obtain a color image of a subject.
This imaging element has a multilayer structure formed by a semiconductor process over a silicon substrate. An opto-electrical conversion part that converts light into an electrical signal by opto-electrical conversion is formed over the silicon substrate. The opto-electrical conversion part is a photodiode or another such light receiving element. An insulating layer is formed over a silicon substrate on which an opto-electrical conversion part has been formed, and a circuit that transmits electrical signals and a mask that blocks light from being incident on the circuit are formed within this insulating layer. A color filter and a micro-lens are formed in that order of the insulating layer. The imaging element formed by this process is what is known as a front illumination type of imaging element, in which light that has passed through the insulating layer is incident on the opto-electrical conversion part.
The circuit formed at the imaging element is provided along the boundary of the opto-electrical conversion part, and is formed fine so that the light receiving surface area of the light receiving element can be as wide as possible. However, some of the light will be blocked by the mask, so in actual practice the proportion of the total surface area of the light receiving element accounted for by the opto-electrical conversion part (the open ratio) will be about 60% at best.
Meanwhile, better image quality is needed in the field of imaging devices, and the trend is for the number of pixels of an imaging element to increase. If the pixel count is increased without changing the size of the imaging element, the surface area per opto-electrical conversion part will decrease, so the sensitivity of the imaging element ends up being lower than when there are fewer pixels.
In addition to the above-mentioned front illumination type of imaging elements, so-called back illumination imaging elements have also been proposed (see Japanese Laid-Open Patent Application 2005-353996, for example). In the process of manufacturing a back illumination imaging element, an opto-electrical conversion part and an insulating layer are formed in that order on a silicon substrate. The silicon substrate is then removed to form a light receiving face. With a back illumination imaging element, the incident light reaches the opto-electrical conversion part without being blocked by a circuit, so the opto-electrical conversion by the opto-electrical conversion part is more efficient. Thus, using a back illumination imaging element allows the sensitivity of an imaging element to be increased somewhat. With the back illumination imaging element disclosed in Japanese Laid-Open Patent Application 2005-353996, a color filter is disposed on the light receiving face side in order to obtain a color image. A silicon dioxide layer is formed as a protective film between the color filter and the opto-electrical conversion part in order to protect the opto-electrical conversion part.
A technique whereby a plurality of imaging elements are superposed has been proposed in an effort to subject light to opto-electrical conversion more efficiently (see Japanese Laid-Open Patent Application 2005-252411 or Japanese Laid-Open Patent Application 2008-227250, for example). Light is incident on the plurality of imaging elements, and each of these imaging elements performs opto-electrical conversion, allowing the light to be utilized more efficiently. The solid state imaging device disclosed in Japanese Laid-Open Patent Application 2005-252411 has a first imaging element and a second imaging element provided with a color filter. With this solid state imaging device, the first imaging element is affixed on the color filter side of the second imaging element. The composite solid state imaging device disclosed in Japanese Laid-Open Patent Application 2008-227250 has a first imaging element provided with a color filter, and a second imaging element. With this composite imaging element, the second imaging element is superposed via a spacer on the opposite side of the first imaging element from the color filter.
However, when a plurality of imaging elements are affixed with their positions staggered, as with the solid state imaging device disclosed in Japanese Laid-Open Patent Application 2005-252411, light is blocked by the circuit provided to the first imaging element, and it is possible that the second imaging element will not receive enough light. Thus, with a composite imaging element in which a plurality of imaging elements are superposed, there is the risk that high sensitivity will not be obtained, depending on the layout of the imaging elements.
Also, with the composite solid state imaging element disclosed in Japanese Laid-Open Patent Application 2008-227250, the light is attenuated as it passes through the color filter provided to the first imaging element, so even though a plurality of imaging elements are used, there is the risk that sensitivity will still be inadequate.